Magnetization plateaus in dimerized spin-ladder arrays

We investigate the ground-state magnetization plateaus appearing in spin-1/ 2 two-leg ladders built up from dimerized antiferromagnetic Heisenbeg chain s and dimerized zig-zag inteichain couplings. Using both Abelian bosonizati on and Lanczos methods we find that the system yields rather unusual platea us and exhibits massive and massless phases for specific choices or "tuning " of exchange interactions. The relevance of this behavior in the study of NH4CuCl3 is discussed.